Display device

ABSTRACT

The invention relates to an active matrix display device comprising a display with a plurality of display pixels, a data input for receiving a data signal and a controller for distributing said data signal over said display pixels to generate an image on said display with an overall brightness value for each display pixel during at least one frame period. The device is adapted to divide said frame period for at least one subset of said display pixels such that said display pixels of said at least one subset have at least a light output at a first non-zero brightness level during a first sub-period of said frame period and at a second non-zero brightness level during a second sub-period of said frame period, the time averaged sum of said brightness levels being substantially equal to said overall brightness level.

The invention relates to an active matrix display device comprising:

a display with a plurality of display pixels;

a data input for receiving a data signal;

a controller for distributing said data signal over said display pixelsto generate an image on said display with an overall brightness valuefor each display pixel during at least one frame period.

The display preferably is an emissive display, comprising for examplepolymer light emitting diodes (PLEDs) or small molecule light emittingdiodes (SMOLEDs), or alternatively emissive inorganic electroluminescentelements or field emission devices, or a light shutter display such asan active matrix liquid crystal display (LCD), an electrophoreticdisplay, an electrowetting display or an electrochromic display.

Display devices of the hold type are known to suffer from sample/holdeffects. These effects arise from the fact that in every frame period anew image may be displayed at the start of the frame period (sample),whilst in the remainder of the frame period (typically 16 ms for 60 Hzoperation) the image remains visible on the display (hold). This effectis experienced by a viewer as a blurred image if moving images aredisplayed.

The image blurring effect can be reduced by operating the display in apulsed mode, wherein the frame period is time-divided in two sub-frames.The picture is displayed during only one of these sub-frames. Thispulsed mode operation, however, is disadvantageous in that highbrightness levels are difficult to achieve.

US2002/0003520 discloses a hold type display device which holds abrightness of the antecedent picture until the subsequent signal isinputted to a pixel, wherein a frame displaying one picture is timedivided into multiple sub-frames and the brightness of the subsequentsub-frame is attenuated at a designated ratio according to thebrightness of the inputted picture. The thus obtained display deviceprevents a moving picture from being unclear and blurred and controlsthe lowering of the brightness in of the picture.

The prior art display device is disadvantageous in that the device isnot flexible as to the application, i.e. the display device is notadapted to cope with a variety of situations that may be encountered.

It is an object of the invention to provide a display device that ismore flexible.

This object is achieved by an active matrix display device wherein saiddevice is adapted to divide said frame period for at least one subset ofsaid display pixels such that said display pixels of said at least onesubset have at least a light output at a first non-zero brightness levelduring a first sub-period of said frame period and at a second non-zerobrightness level during a second sub-period of said frame period, thetime averaged sum of said brightness levels being substantially equal tosaid overall brightness level. The thus obtained display device is moreflexible in that particular subsets of display pixels can have lightoutputs at different brightness levels within the frame time. Thesesubsets can be defined by one or more criteria adapted to the specificsituation encountered, including the colours of the display pixels for acolour display, the display pixels belonging to a specific area on thedisplay and/or the total time during which a display pixel has had alight output. Such subsets may e.g. be appropriate for situationswherein only part of the display is likely to suffer from sample/holdeffects or where chances of degradation of a set of display pixels islikely because of heavy use.

It is noted here that the light output from the display pixels can beobtained in several ways, including the emission of light by an emissiveelement of an emissive display and the transmission or reflection oflight by a display pixel in a light shutter type display.

In a preferred embodiment, the first brightness level exceeds the secondbrightness level and/or the first sub-period and the second sub-periodare of different duration, such as a first sub-period of shorterduration than said second sub-period. Sample/hold effects are reducedfurther if the display pixel emits the first brightness level for lessthan 50% of the frame period if the first brightness level exceeds thesecond brightness level.

In an embodiment of the invention the active matrix display device isadapted to provide a select signal for selecting said display pixels ofsaid subset, said select signal comprising at least a first selectsignal triggering said first sub-period and a second select signaltriggering said second sub-period. In this embodiment the display pixelsof the subset are addressed more than once for each frame period toaccomplish the sub-periods by supplying various addressing pulses tothese display pixels. In this way the distribution of the brightnesslevels between the sub-periods can be chosen freely as long as the sumof the brightness levels for the various sub-periods yields the overallbrightness.

In an embodiment of the invention the display pixels comprise currentemissive elements, such as PLED or SMOLED elements, driven by driveelements and said device is adapted to vary a voltage for said driveelements, such that said at least one subset of current emissiveelements is driven to at least said first brightness level during saidfirst sub-period and said second brightness level during said secondsub-period. In yet another embodiment of the invention, the display isan active matrix liquid crystal display, said device comprising abacklight and being adapted to control said backlight such that saidlight output of said display pixels of said at least one subset yieldssaid first brightness level during said first sub-period (F1) and saidsecond brightness level during said second sub-period (F2). In contrastto the previously discussed embodiment concerning the multipleaddressing, these embodiments do not require substantial processing ofthe data signal. The display pixels can be dimmed in the secondsub-period whilst only addressing the display pixels once.

In a preferred embodiment the active matrix display comprises a colourdisplay and said backlight is a LED-backlight or a colour sequentialbacklight. Such a backlight provides the possibility to separate red,green and blue coloured light and provide individual control of theselight components as a light input for the display pixels. As such, thecolour subset becomes an option for coloured LCD displays.

In an embodiment of the invention the active matrix display device isadapted to generate said light output such that said second brightnesslevel has a brightness that is 30% or less than said first brightnesslevel. Perception studies have revealed that viewers experience anacceptable reduction of motion blur artefacts even if the secondbrightness level yields a brightness of 30% of the brightness obtainedin the first sub-period.

It should be appreciated that the embodiments, or aspects thereof, maybe combined.

The invention further relates to an electric device comprising a displaydevice as described in the previous paragraphs. Such an electric devicemay relate to handheld devices such as a mobile phone, a PersonalDigital Assistant (PDA) or a portable computer as well as to devicessuch as a Personal Computer, a television set or a display on e.g. adashboard of a car.

British Patent Application No 0316862.2 describes an active matrixdisplay device containing current driven emissive elements. The frameperiods for the display are divided in a first sub-period during whichthe emissive element carries a first non-zero current and a secondsub-period during which the emissive element carries a second non-zerocurrent. The first and second non-zero current substantially yield theoverall brightness for each display pixel. The patent application doesnot describe a display device that is adapted to activate subsets ofdisplay pixels and thus cannot obtain the flexibility provided by thecurrent invention.

The invention will be further illustrated with reference to the attacheddrawings, which show preferred embodiments according to the invention.It will be understood that the invention is not in any way restricted tothese specific and preferred embodiments.

FIG. 1 shows an electric device comprising an active matrix displaydevice according to an embodiment of the invention;

FIG. 2 shows a light output profile according to an embodiment of theinvention;

FIGS. 3A-3C show various examples of subsets of display pixels accordingto different embodiments of the invention;

FIG. 4 shows a schematical illustration of an active matrix displaydevice shown in FIG. 1, comprising a display with current emissiveelements according to an embodiment of the invention;

FIG. 5 shows a voltage addressed display pixel for a display shown inFIG. 4, according to an embodiment of the invention;

FIG. 6 shows a current addressed current mirror display pixel for adisplay shown in FIG. 4, according to an embodiment of the invention;

FIG. 7 shows a schematical illustration of an active matrix displaydevice along cross-section A-A in FIG. 1, comprising a liquid crystaldisplay according to an embodiment of the invention;

FIG. 8 shows a schematical illustration of an active matrix displaydevice along cross section A-A in FIG. 1, comprising an electrophoreticdisplay.

FIG. 1 shows an electric device 1 comprising an active matrix display 2having a plurality of display pixels 3 arranged in a matrix of rows 4and columns 5. The display 2 preferably is an emissive display,comprising display pixels 3 containing polymer light emitting diodes(PLEDs) or small molecule light emitting diodes (SMOLEDs), or a lightshutter display such as an active matrix liquid crystal display (LCD),an electrophoretic display, an electrowetting display or anelectrochromic display. The display 2 may alternatively be a displaywith emissive inorganic electroluminescent elements or a field emissiondisplay device. The display 2 may be a large display as motion artefactsare generally most visible on such large displays. An example of asubset S of display pixels 3 is indicated.

FIG. 2 shows an illustration of a light output profile P according to anembodiment of the invention. The light output profile P is obtained froma subset S of display pixels 3 in a manner that will be described andillustrated below. The frame period F is divided in a first sub-periodF1 and a second sub-period F2. The display pixels 3 of the subset S havea light output L at a first brightness level L1 during the firstsub-period F1 and at a second brightness level L2 during the secondsub-period F2. The time averaged sum of the brightness levels L1 and L2is substantially equal to the overall brightness level, i.e. theinvention achieves the same overall brightness level (=integral areaunder the profile P) by maintaining a finite non-zero second brightnesslevel L2 such that a first brightness level L1 can be reduced incomparison with the high peak brightness level Lh of the dashed profileQ representing a conventional reduced duty cycle profile for a display.This allows for the adaptation of the light output L for only a subset Sof display pixels 3 to e.g. modify the quality wherein the image can bedisplayed. Perception studies have revealed that motion blur artefactsare perceived as being reduced even if L2 is 30% or less than L1.

It is noted that the profile P in FIG. 2 only illustrates a simpleexample. More complex profiles may be generated, including varying lightoutputs L1 in subsequent frame periods F, the light output L2 being keptat a fixed or variable ratio or a stable level for the sub-period F2,and multiple sub-periods F1, F2, . . . Fn having corresponding non-zerobrightness levels L1, L2, . . . Ln. The sub-periods F1 and F2 may be ofdifferent duration.

FIGS. 3A-3C show three examples of subsets S of display pixels 3 forwhich the frame period F is time-divided in a first sub-period F1 duringwhich L1 is obtained and a second sub-period F2 during which L2 isobtained.

FIG. 3A shows a colour display 2 that may e.g. comprise sub-pixels 3applying red (white circles), green (light-grey circles) or blue(dark-grey) emissive elements. The subset S consists e.g. of the red (R)and blue (B) display pixels 3 as green display pixels 3 may e.g. be morelight efficient and exhibit extremely long lifetimes. According to anembodiment of the invention the subset S of R and B display pixels 3yields a light output L in accordance with the profile P of FIG. 2,whereas the G display pixels 3, for which the driving method is lessneeded, yield a light output in accordance with the profile Q. In thisway driving is simplified and the sample/hold effect is further reduced.

In another example the subset S consists of said green sub-pixels G 3only, while the R and B display pixels 3 are driven in a non-pulsedmode, i.e. the R and B display pixels 3 yield a constant light outputover the frame period F. In this way, driving is further simplified, thedisplay lifetime is extended as the red and blue emissive elements donot experience high currents and an acceptable image perception ismaintained reducing the sample/hold effects of the (dominant) greenelements.

FIG. 3B shows an example wherein the subset S is defined as the displaypixels 3 of a particular area A. This area A may e.g. be a windowdisplaying a video on a stationary background. As sample/hold effectsonly occur in the area A of moving images in such a situation, anembodiment of the invention allows a division of the frame period F intoframe sub-periods F1 and F2 with a first brightness level L1 and asecond brightness level L2 respectively, only for the area A. It isnoted that area A may vary in position on the display and more than onearea A may be present. The display pixels not belonging to the subset Smay be driven in another mode, e.g. to yield a constant light output.

FIG. 3C shows an example wherein the subset S is defined as the displaypixels 3 that output light for the longest times. If the display 2 e.g.continually displays icons I, the display pixels 3 displaying this iconI have a long on-time. If a display 2 comprises current emissiveelements it is known that these elements degrade in performance withtime. This degradation is slowed down by defining these pixels as thesubset S and dividing the frame period F in a first sub-period F1 duringwhich a first brightness level L1 is output and a second sub-period F2during which the second brightness level L2 is output. Alternatively, tofurther slow down the degradation, the icon pixels could be driven at aconstant light output whilst other pixels could be driven with a profileP in FIG. 2.

It is explicitly noted here that other types of subsets S can beenvisaged without departing from the true spirit of the invention. Nextseveral examples of types of displays 2 will be discussed as well assome examples for the implementation of the embodiments of theinvention, discussed with respect to FIGS. 2 and 3, in these types ofdisplays 2.

FIG. 4 shows a schematical illustration of an active matrix displaydevice 6, comprising an OLED display 2 of the electric device 1 as shownin FIG. 1 having current emissive elements shown in detail in FIG. 5 andFIG. 6). The display 2 comprises a display controller 7, includingamongst others a row selection circuit 8 and a data register 9. A datasignal, comprising information or data such as (video) images to bepresented on the display 2, is received via data input 10 by the displaycontroller 7. The data are written to the appropriate display pixels 3from the data register 8 via data lines 11. The selection of the rows 4of the display pixels 3 is performed by the row selection circuit 8 viaselection lines 12, controlled by the display controller 7.Synchronization between selection of the display pixels 3 and writing ofthe data to the display pixels 3 is performed by the display controller7. Moreover the display controller 7 controls the power supply of thedisplay pixels 3 via power lines 13, 15.

FIGS. 5 and 6 show two examples of circuit arrangements for the displaypixels 3 for the active matrix display device 6.

FIG. 5 shows a voltage addressed circuit arrangement for a display pixel3 comprising an addressing transistor T1, a storage capacitor C and adrive element T2 for applying a driving signal to a current drivenemissive element 14. The transistor T1 is arranged to receive a selectsignal over the line 12 from the display controller 7. T1 passes thedata signal for a particular frame to the gate of T2 if an appropriateselect signal is received. T2 may be a p-Si thin film transistor (TFT)and the current driven emissive element 14 may be a light emittingdiode, such as an polymer light emitting diode (PLED) or small moleculeorganic light emitting diode (SMOLED). The current emissive element 14may e.g. emit red, blue or green light. One of the plates of thecapacitor C and the source electrode of T2 are connected to the powersupply line 13. A voltage may be applied to T2 as well via line 15.

If T2 is biased in saturation it behaves as a constant current source,passing a current which is proportional to μ_(fe).(V_(GS)−V_(T))² whereV_(GS) is the gate-source voltage of T2, V_(T) the threshold voltage,and μ_(fe) is the field effect mobility of T2. This constant current isthen driven through the emissive element 14 which is connected to T2.Thus, the current source is programmed by setting the voltage on thegate of T2. This is conventionally achieved during a short addressingtime of e.g. 25 μs by turning on T1 via line 12 and transferring thesignal voltage from the data register 9 to the gate of T2. T1 is thenswitched off, and the programmed voltage is held on the gate of T2 forthe rest of the frame period F. The storage capacitor C preventsappreciable discharge of this node via leakage through T1, thus forminga memory to allow continuous LED current while the other rows 4 of thedisplay 2 are selected sequentially. It is noted that voltage addresseddisplay pixels 3 are known in many variants that may employ furthertransistors. Such variants fall under the scope of the presentinvention.

Another category of display pixel circuits are the current addresseddisplay pixel circuits 3 shown in FIG. 6. The driving transistor T2 isused in both addressing the display pixel 3 and in driving the emissiveelement 14. T2 preferably is a short channel TFT for reasons describedin the co-pending application British Patent Application No 0316862.2(“Display device”) of the applicant that is incorporated in the presenttext by reference with respect to this feature. The data input signal isapplied as a current rather than as a voltage over the line 11,indicated by the current source 1. During the addressing period thedriving transistor T2 is diode-connected by the transistor T4 toaddressing transistor T1, and the emissive element 14 is isolated fromthe circuit by the transistor T3. During this addressing period the datainput current is forced through T2 while the capacitor C is charged toreach the associated gate-source voltage V_(GS) for T2. Now, by openingT1 and T4 and by closing T3, the drain current is fed to the emissiveelement 14. The memory function of the capacitor C assures the currentto be a perfect copy of the data input current received over line 11.

In operation the display controller 7 may generate a select signal 18 inan embodiment of the invention and provide this select signal 18 overthe select lines 12 to a subset S of display pixels. The select signal18 comprises a first select signal 18′ triggering the first sub-periodF1 and a second select signal 18″ triggering the second sub-period F2shown in FIG. 2. The display pixel 3 is thus voltage- or currentaddressed more than once for each frame period F by supplying variousaddressing pulses to the selecting means T1 and T1, T3, T4 respectively.The display controller 7 may determine the display pixels 3 belonging tothe subset S, process the data signal received over input 10 anddistribute this data signal for the frame period F as a first brightnesslevel L1 output during the first sub-period F1 and a second brightnesslevel L2 output during the second sub-period F2. Double addressing canalso be accomplished by providing an additional addressing line 12 foreach display pixel 3 to save power and addressing time.

In another embodiment the display controller 7 may vary the voltage forthe drive element T2 such that during the first sub-period F1 a firstcurrent I1 is applied to the emissive element 14 to yield the brightnesslevel L1 and subsequently, during the sub-period F2, a second current I2to yield the brightness level L2. This can be performed by controllingthe power supply over line 13 or 15 for the subset S of display pixels3, determined by the controller 7 for the situation encountered. If e.g.the display 2 is a colour display the display device 6 may accomplishthe situation as was described for FIG. 3A as follows.

Typically a coloured display pixel 3 comprises a red, green and blueemissive element 14 whereas the display device 6 is adapted to drivethese emissive elements independently of each other. It was observedthat some of the emissive elements 14 are more efficient than others interm of light output and moreover that the lifetime of emissive elementsmay vary considerably from colour to colour. Particularly green emissiveorganic light emitting diodes yield a high light output L, i.e. morelight per electron, and show extremely long lifetimes. Therefore in anembodiment of the invention the display 2 comprises a subset S ofdisplay pixels 3 or emissive elements 14 and said device 6 supplies afirst non-zero current and a second non-zero current to only the red andblue display pixels 3 of the subset S by addressing these pixels 3 twiceapplying select signals 18′and 18″ in the frame period F. Alternativelythe voltages for the drive elements T2 can be varied via lines 13 or 15of the R and B display pixels 3 of the subset S.

As another example only the green emissive elements 14 are manipulated,e.g. by varying the voltage for the drive elements T2 of these G displaypixels 3 during the frame period F, while the red and blue emissiveelements 14 are simply driven continuously at a constant light outputduring this frame period F. In this way the driving of the display 2 issimplified even further and the lifetime of the display 2 is extended asthe red and blue emissive elements 14 experience no high current pulses.Sample/hold effects are reduced by perception by solely manipulating thedominant green emissive elements 14.

A similar operation may be applied for emissive types of display 2 increating the subsets S shown in FIGS. 3B and 3C. The controller 7 maye.g. register the total time during which a display pixel 3 has been onand define a threshold time above which the display pixel is determinedas belonging to the subset S. The light output L of these display pixels3 of the subset S should yield the light output profile P to prolong thelifetime of these display pixels 3.

FIG. 7 schematically shows an active matrix display device, 6 alongcross-section A-A in FIG. 1, having an LCD display 2 with display pixels3. Light may be generated from a backlight 20. The backlight 20 may e.g.be a normal white backlight, a backlight employing light emitting diodes(especially useful to employ coloured subsets S as shown in FIG. 3A) ora sequential colour backlight. Alternatively the backlight 20 may bereplaced by a reflector reflecting ambient light to the display pixels3. The display 2 comprises a grid of conductive address lines 21 anddrive lines 22 connected to active circuitry (not shown) to control andswitch liquid crystal elements in part 23 of the display 2 by acontroller 7. The controller 7 determines the polarization states of theliquid crystal elements in the part 23 and, as a consequence of this,the light output L of the display pixels 3. The controller 7 may alsocontrol the backlight 20 over line 24 depending on the kind of backlightemployed. Other components, such as e.g. conventionally applied glassplates, polarizing filters and/or colour filters are not shown in FIG. 7for reasons of clarity.

In an embodiment of the invention the display device 6 of FIG. 7 mayoperate as follows. The controller 7 monitors a video data signal at thedata input 10 to be displayed in a window A (see FIG. 3B). The displaypixels 3 belonging to this window A are defined as the subset S asdetermined by the controller 7. The light output profile P for thedisplay pixels 3 of the subset S is suitable to decrease the perceptionof motion blur artefacts, as was previously described. This light outputprofile P may be obtained by dividing the frame period F in sub-periodsF1, F2 by e.g. addressing the display pixels 3 of the subset S twiceover line 21. The polarization state of the liquid crystal elements iscontrolled via the lines 22 such that the output brightness level L1 isobtained during the first sub-period F1, while the polarization state isswitched to obtain the second brightness level L2 during the secondframe period F2. The display pixels 3 not belonging to the subset S maye.g. be driven such that their light output profile is constant.Alternatively for a colour LCD 2, the subset S can be defined by solelythe green display pixels 3. The green display pixels may be driven toyield the profile P, while e.g. the blue display pixels 3 may be drivento yield the profile Q and the red display pixels yield a continuouslight output L over the entire frame period F. These embodiments providethe advantage that less signal processing is needed in the controller 7compared to the display device of US 2002/0003520.

In yet another embodiment of the invention the controller 7 controls thebacklight 20 over line 24 to yield the light output profile P for theLCD 2 shown in FIG. 2. If the backlight 20 is a LED backlight or acoloured sequential backlight, the controller 7 may control the colourand the brightness level L1, L2 of the light that is input to thedisplay 2 whilst the display pixels are only addressed once.

FIG. 8 shows another embodiment, as part of cross-section A-A as definedin FIG. 1, of a display device 6 comprising an electrophoretic display 2to which the invention can be applied. The display device 6 comprises afirst substrate 30, a second opposed substrate 31 and a plurality ofdisplay pixels 3. Preferably, the display pixels 3 are arranged alongsubstantially straight lines in a two-dimensional structure as shown inFIG. 1. Other alternatives include e.g. a honeycomb structure. Anelectrophoretic medium 32, having charged particles 33, is presentbetween the substrates 30 and 31. The first substrate 30 has for eachdisplay pixel 3 a transparent first electrode 34, and a second electrode35. The second electrode is covered with a non-transparent black matrixB. A controller 7 comprises a data signal input 10 for receiving thedata signal. The controller 7 distributes the data signal over thedisplay pixels 3 to generate an image on the display 2 via theconductive lines 36,37, active circuitry (not shown) in the substrate 30and the electrodes 34, 35. The charged particles 33 are able to occupyextreme positions near the electrodes 34,35 and intermediate positionsin between the electrodes 34,35. In this way grey levels can be obtaineddepending on the voltage applied over the electrodes 34,35. Each displaypixel 3 has a brightness level L determined by the position of thecharged particles 33 between the electrodes 34,35 for displaying thepicture or image. As an example, the electrophoretic medium 32 comprisesnegatively charged black particles 33 in a transparent fluid, situatedabove a white background. When the charged particles 33 are in a firstextreme position, e.g. near the first electrode 34, as a result of thepotential difference being e.g. 15 Volts, the appearance of the displaypixel 3 is e.g. black. Here it is considered that the display pixel 3 isobserved from the side of the second substrate 31. When the chargedparticles 33 are in a second extreme position, i.e. near the secondelectrode 35, as a result of the potential difference being of oppositepolarity, i.e. −15 Volts, the appearance of the display pixel 3 iswhite. When the charged particles 33 are in one of the intermediatepositions, i.e. in between the electrodes 34,35 the display pixel 3 hasone of the intermediate appearances, e.g. light grey and dark grey,which are grey levels between white and black. The electrophoreticdisplay 2 may be used with the backlight options discussed above.

In operation the electrophoretic display 2 may e.g. display a movingimage in a window A. The display pixels 3 positioned in this window Aare defined as constituting the subset S for which the frame period F isto be time divided in a sub-period F1 and F2. This division may beaccomplished by double addressing of the display pixels 3 via the line36. The light output profile P as shown in FIG. 2 is obtained byapplying a first drive voltage to the charged particles 33 during thefirst sub-period F1 to obtain the first brightness level L1 and a seconddrive voltage during the second sub-period F2 to obtain a light output athe second brightness level.

1. Active matrix display device (6) comprising: a display (2) with aplurality of display pixels (3); a data input (10) for receiving a datasignal; a controller (7) for distributing said data signal over saiddisplay pixels (3) to generate an image on said display (2) with anoverall brightness value for each display pixel (3) during at least oneframe period (F), wherein said device (6) is adapted to divide saidframe period (F) for at least one subset (S) of said display pixels (3)such that said display pixels (3) of said at least one subset (S) haveat least a light output (L) at a first non-zero brightness level (L1)during a first sub-period (F1) of said frame period (F) and at a secondnon-zero brightness level (L2) during a second sub-period (F2) of saidframe period (F), the time averaged sum of said brightness levels(L1,L2) being substantially equal to said overall brightness level. 2.Active matrix display device (6) according to claim 1, wherein saiddisplay (2) is a colour display and said subset (S) is defined by colour(R, G, B).
 3. Active matrix display device (6) according to claim 1,wherein said device (6) is adapted to determine one or more particularareas (A) of said display and said subset is defined by said areas. 4.Active matrix display device (6) according to claim 1, wherein saiddevice (6) is adapted to determine the total time during which saiddisplay pixels (3) have had a light output and said subset (S) isdefined by said total time.
 5. Active matrix display device (6)according to claim 1, wherein said first brightness level (L1) exceedssaid second brightness level (L2).
 6. Active matrix display device (6)according to claim 1, wherein said first sub-period (F1) has a shorterduration than said second sub-period (F2).
 7. Active matrix displaydevice (6) according to claim 1, wherein said device (6) is adapted tosupply a select signal (18) for selecting said display pixels (3) ofsaid subset (S), said select signal (18) comprising at least a firstselect signal (18′) triggering said first sub-period (F1) and a secondselect signal (18″) triggering said second sub-period (F2).
 8. Activematrix display device (6) according to claim 1, wherein said displaypixels (3) comprise current emissive elements (14) driven by driveelements (T2) and said device (6) is adapted to vary a voltage (13;15)for said drive elements (T2) such that said at least one subset (S) ofcurrent emissive elements (14) is driven to at least said firstbrightness level (L1) during said first sub-period (F1) and said secondbrightness level (L2) during said second sub-period (F2).
 9. Activematrix display device (6) according to claim 1, wherein said display (2)is an active matrix liquid crystal display, said device (6) comprising abacklight (20) and being adapted to control said backlight (20) suchthat said light output (L) of said display pixels (3) of said at leastone subset (S) yields said first brightness level (L1) during said firstsub-period (F1) and said second brightness level (L2) during said secondsub-period (F2).
 10. Active matrix display device (6) according to claim9, wherein said display (2) is a colour display and said backlight (20)is a LED-backlight or a colour sequential backlight.
 11. Active matrixdisplay device (6) according to claim 1, wherein said device (6) isadapted to generate said light output (L) such that said secondbrightness level (L2) has a brightness that is 30% or less than saidfirst brightness level (L1).
 12. Electronic device (1) comprising anactive matrix display device (6) according to claim 1.